Many systems require a hydrogen supply for operation as, for example, fuel cells. If the fuel cell system is in a motor vehicle, for example, the hydrogen utilized by the fuel cell system is stored in, preferably, liquid form in a liquid hydrogen storage system comprised of a liquid hydrogen storage tank and associated components such as, for example valves and pipes, located within the motor vehicle. In order to maintain the hydrogen in a substantially liquid form in the liquid hydrogen storage tank, the liquid hydrogen must be kept at cryogenic temperatures, temperatures below minus two hundred fifty degrees Celsius. When it is necessary to refill the liquid hydrogen storage tank located within the motor vehicle at, for example, a liquid hydrogen tank station, liquid hydrogen flows from the liquid hydrogen tank station to the liquid hydrogen storage tank through connector piping of a filling connector of the liquid hydrogen storage system connecting the liquid hydrogen storage tank to the liquid hydrogen tank station. At the start of the refilling process, the connector piping of the liquid hydrogen storage system connecting the liquid hydrogen storage tank to the liquid hydrogen tank station may be at higher temperature than the liquid hydrogen being transported from the liquid hydrogen tank station to the liquid hydrogen storage tank. The higher temperature of the connector piping causes a substantial portion of the liquid hydrogen being transported to evaporate. The gaseous hydrogen produced by evaporation prevents the liquid hydrogen storage tank from filling with liquid hydrogen and flows back to the liquid hydrogen tank station after passing through the liquid hydrogen storage tank. After a few minutes of refilling, the connector piping becomes cooled by the liquid hydrogen being transported to a temperature such that liquid hydrogen reaches the liquid hydrogen storage tank and the liquid hydrogen storage tank can then be filled with liquid hydrogen.
FIG. 1 is an example of a prior art liquid hydrogen storage system 100 suitable for use with a fuel cell system in a motor vehicle. The liquid hydrogen storage system 100 is composed of liquid hydrogen storage tank 102, cryo-block 104, filling connector 104′, cryo-coupling valve 106, heat exchanger 108, shut-off valve 110, safety valve 112, and boil-off valve 114. Liquid hydrogen storage tank 102 contains hydrogen; a portion 116 thereof in liquid form and a portion 118 thereof in gaseous form along with level sensor 120 and pipes 122, 124. Cyro-block 104 consists of gas valve 126, liquid valve 128, filling valve 130, and pipes 132-140. For fuel cell system operation, gaseous and liquid hydrogen in discharge pipe 140 flows through energized heat exchanger 108 into pipe 144 through shut-off valve 110, which is connected to a fuel cell system 146. Filling connector 104′ consists of piping 145 in the form of pipes 138″ and 142 between cyro-block 104 and cryo-coupling valve 106. Cryo-coupling valve 106 connects to a liquid hydrogen tank station (see FIG. 3B) to refill the liquid hydrogen storage tank 102.
FIG. 2A depicts valves 110, 126, 128, and 130 in the closed position whereas FIG. 2B depicts valves 110, 126, 128, and 130 in the open position. FIG. 3A depicts the closed position of cryo-coupling valve 106 when the liquid hydrogen storage system 100 is not connected to a liquid hydrogen tank station; while FIG. 3B depicts the open position of cryo-coupling valve 106 when the liquid hydrogen storage system 148 is connected to a liquid hydrogen tank station via coupling 302.
Liquid hydrogen storage system 100 includes a discharge mode of operation and a refilling mode of operation, wherein when utilized in a motor vehicle the discharge mode of operation as two sub-modes, parked and driving modes. In parked mode, all valves 106, 110, 126, 128, and 130 are closed and heat exchanger 108 is not energized.
In driving mode, if the pressure in liquid hydrogen storage tank 102 is above a predetermined pressure, gaseous hydrogen 118 flows into pipes 122 and 132 through open gas valve 126 into pipe 132′ and discharge pipe 140 to energized heat exchanger 108. After passing through energized heat exchanger 108, gaseous hydrogen flows into pipe 144 and through open shut-off valve 110 to the fuel cell system 146. Valves 106, 128 and 130 are in the closed position during this time.
Otherwise, in driving mode, if the pressure in liquid hydrogen storage tank 102 is below a predetermined pressure, liquid hydrogen 116 flows into pipes 124, 136, and 134 through open liquid valve 128 into pipe 134′ and discharge pipe 140 to energized heat exchanger 108. After passing through energized heat exchanger 108, gaseous hydrogen flows into pipe 144 and through open shut-off valve 110 to the fuel cell system. Valves 106, 126 and 130 are in the closed position during this time.
In refilling mode, there will either be a small amount or no liquid hydrogen 116 in liquid hydrogen storage tank 102. Hence, liquid hydrogen storage tank 102 will contain substantially gaseous hydrogen 118. A liquid hydrogen tank station 148 is connected to open cryo-coupling valve 106 via coupling 302 as depicted in FIG. 3B. Liquid hydrogen flows from the liquid hydrogen tank station 148 into the filling connector 104′ through the connector piping 145 via pipe 138″, through pipe 138′, through open filling valve 130, and through pipes 138, 136 and 124 to the liquid hydrogen storage tank 102. Gaseous hydrogen 118 flows into pipes 122 and 132 through open gas valve 126 through pipe 132′, into the filling connector 104′ through the connector piping 145 via pipe 142, and through cryo-coupling valve 106 into coupling 302 back to the liquid hydrogen tank station. Heat exchanger 108 is de-energized and shut-off valve 110 is closed during this mode.
At the start of the refilling process, the connector piping 145 (pipes 138″ and 142) of the filling connector 104′ are at a higher temperature than the liquid hydrogen being transported from the liquid hydrogen tank station to the liquid hydrogen storage tank 102. The higher temperature of the connector piping causes a substantial portion of the liquid hydrogen being transported to evaporate. The gaseous hydrogen produced by evaporation flows through pipes 138″ and 138′, open filling valve 130, and through pipes 138, 136 and 124 and enters the liquid hydrogen storage tank 102 as gaseous hydrogen 118 and prevents the liquid hydrogen storage tank from filling with liquid hydrogen, whereupon the gaseous hydrogen returns to the liquid hydrogen tank station as previously described. After a few minutes of refilling, the connector piping 145 (pipes 138″ and 142) are cooled sufficiently by the hydrogen being transported so as to be at a cryogenic temperature such that liquid hydrogen reaches the liquid hydrogen storage tank 102 as previously described and the liquid hydrogen storage tank can then be filled with liquid hydrogen.
The gaseous hydrogen produced through evaporation of the transported liquid hydrogen from the liquid hydrogen tank station due to the temperature of the connector piping 145 flowing back to the liquid hydrogen tank station, as previously described, may be recovered or just vented to the atmosphere. If the gaseous hydrogen is recovered, energy must be expended to re-liquefy the gaseous hydrogen. If the gaseous hydrogen is vented to the atmosphere, it is lost. Hence, if the amount of gaseous hydrogen produced by the refilling process through evaporation can be reduced, a significant amount of energy and hydrogen can be saved.
Accordingly, what is needed in the art is a method of reducing gaseous hydrogen losses when the liquid hydrogen storage tank is refilled with liquid hydrogen.